The fluid catalytic cracking (FCC) process is a major source of SOx and NOx gas emissions in petroleum refining. In the FCC process, heavy hydrocarbon fractions are cracked to produce lighter, valuable products (gasoline, distillate, and C2-C4 olefins and saturated hydrocarbons). The cracking process takes place in the riser section of a FCC unit, where the hydrocarbon fractions are contacted with a FCC catalyst and other additive particles at elevated temperature. As the endothermic cracking reactions occur, coke (containing carbon, sulfur and nitrogen, among other components) is deposited onto the FCC catalyst and any additive particles. These particles are carried from the FCC unit's reactor to the catalyst regenerator, and the coke is burned off to form carbon monoxide, carbon dioxide, and a wide-range of gaseous sulphur and nitrogen species, e.g., SO2, SO3, COS, H25, N2, NO, N2O, NO2, NH3, and HCN.
The exact composition of these gases in the flue gas depends upon the detailed reaction conditions in the regenerator. For example, under full-burn conditions (excess oxygen used in combustion of coke) the main species are SO2, SO3, N2 and NO. Under partial-burn conditions (sub-stoichiometric oxygen levels) much higher levels of the “reduced” S and N species (COS, H2S, NH3, HCN) may also be present. Most partial-burn FCC units will have a CO-boiler (COB) downstream to convert the CO to CO2 in order to control CO emissions and recover the energy for steam production. Most of the reduced S and N species in the flue gas are converted to more highly oxidized forms in the CO-boiler. So COS and H2S are converted to SO2 and SO3; and NH3 and HCN are converted to N2, NO, N2O, and NO2. Therefore, in both full-burn and partial-burn operations, the flue gas contaminants are predominantly sulfur oxide gases (e.g., SO2 and SO3 which are often collectively referred to as “SOx” gases) and nitrogen oxide gases (e.g, NO, N2O, NO2 which are often collectively referred to as “NOx” gases). Other species are only present at much lower concentrations. Unless captured or removed, the hydrogen cyanide would be emitted to the atmosphere along with other flue gases.
Various processes have been taught to remove or reduce the amount of these gases discharged to atmosphere from a FCC unit. One method to remove NOx gases has been to use ammonia as a reductant to reduce the NOx to from nitrogen. See, for example, U.S. Pat. No. 4,778,665 which teaches the reduction of NOx with ammonia in the presence of a crystalline zeolite catalyst. U.S. Pat. No. 4,778,665 teaches that at an appropriate point downstream of the ammonia injection, after reduction of the NOx, the particles are recovered by a cyclone separator or by an electrostatic precipitator, and the flue gas then is discharged.
It is desirable to attain still further improvements in the reduction of NOx gas in the effluent from an FCC unit.